P316: THE ROLE OF TET2 IN (PRE)-LEUKEMIC T-ALL: TET2 TO THE RESCUE?
Authors: S. De Coninck, P. Van Vlierberghe, S. Goossens, B. Lintermans, J. Roels
Journal: HemaSphere (2022)

Background: T-cell acute lymphoblastic leukemia (T-ALL) accounts for 15% of pediatric and 25% of adult ALL patients. Therapy has been intensified over the last decades, leading to gradual improvements in survival. However, 10-15% of pediatric and 50% of adult T-ALL cases relapse and ultimately die because of refractory disease. During malignant transformation of T-cells, a clonal expansion of immature thymic cells is selected through the gradual accumulation of advantageous epigenetic changes and genetic mutations. Long-lived preleukemic stem cells (pre-LSCs) were uncovered as an initiating event in various blood-born cancers, including in mouse models of T-ALL. Advanced next-generation sequencing technology allowed for the identification of these predisposing mutations in preleukemic myelodysplastic syndrome patients before they progress towards acute myeloid leukemia. Substantial enrichment for mutations in regulators of DNA methylation were identified, including loss of TET2. TET2 is an enzyme that catalyzes DNA demethylation and Tet2 knockouts are prone to develop lymphoid malignancies, including T-cell lymphoblastic leukemia/lymphoma. In T-ALL, the role of TET2 remains to be investigated. We hypothesize that TET2 acts as a tumor suppressor in T-ALL, involved in a premalignant clonally expanded pre-LSC population. Aims: We aimed to evaluate the tumor suppressor role of TET2 in the development of preleukemic stem cells that ultimately give rise to T-ALL, and to gain a better understanding of the molecular mechanisms involved in this process of malignant T cell transformation. Methods: We evaluated the expression of TET2 in RNAseq data of primary T-ALL samples and compared this with their normal T cell counterparts. Next, we used a spontaneous T-ALL mouse model, CD2-LMO2tg mice. Already at young age, preleukemic CD2-Lmo2tg thymocytes gain aberrant self-renewal capacity, which allows clonal expansion of an immature thymocyte precursor subpopulation. This T-ALL mouse model was crossed with a R26-Tet2 knock-in mouse model that allows conditional (CD2-icre) Tet2 expression in all lymphocytes, at an early differentiation stage, before T-cell commitment. Results: In T-ALL, TET2 mutations are sporadically identified. However, gene expression profiling of primary T-ALL samples identified a large subset of T-ALLs characterized by a profound loss of TET2 expression compared to healthy T cell precursors (Fig. 1A). These low TET2 levels occur in patients from different T-ALL subtypes and were confirmed in a subset of T-ALL cell lines. To further explore the relevance of TET2 silencing and its potential implications in clonal expansion, preleukemic CD2-Lmo2tg mice were sacrificed at different time points (8 and 24 weeks). Gene expression levels in premalignant T-cells were compared with age matched wild-type control mice. A gradual decrease of Tet2 expression, with a co-occurrent increase of Tet2 promotor methylation, was detected in preleukemic CD2-Lmo2tg mice (Fig. 1B-C), which confirms the important role of Tet2 in pre-LSC. To support the tumor suppressor role of Tet2 in T-ALL, an aging cohort of CD2-Lmo2tg with and without Tet2 overexpression was built. Survival analysis showed that CD2-LMO2tg mice with Tet2 overexpression have an increased latency of T-ALL (Fig. 1D, p=0,03). Image:Summary/Conclusion: Tet2 plays an important role in pre-LSC and can act as a tumor suppressor in a spontaneous T-ALL mouse model, which supports the possible tumor suppressor role of TET2 in a subgroup of T-ALL patients. Loss of TET2 in these patients might be a good predictor for response to DNA methylation inhibitors.